Quantitative Assessment and Visualisation of the Wood and Poly(Lactic Acid) Interface in Sandwich Laminate Composites

Abstract

Fluorescence microscopy was applied to understand adhesion interfaces developed within laminated composite sandwiches formed between poly(lactic acid) (PLA) and wood veneers. Composites formed with maple veneer had greater tensile bond strength when manufactured at 200 °C (10.4 N/mm2) compared to formation at 140 °C (8.7 N/mm2), while significantly lower bond strength was achieved using spruce veneers, at 5.2 and 3.5 N/mm2, respectively. Qualitative and quantitative confocal microscopy assessments revealed differing bondline thicknesses and PLA ingress within the wood ultrastructure. Forming maple veneer composites at 200 °C promoted greater PLA mobility away from the bondline to reinforce the wood–PLA interface and deliver associated greater composite bond strength. The addition of 25% wood fibre to PLA led to fibre alignment and overlap within bondlines contributing to relatively thicker, heterogeneous bondlines. Study outcomes show that the composite processing temperature impacts the adhesion interface and composite performance and will have broad application over veneer overlays, laminates and wood plastic composites (WPCs) using wood, particles or fibres with PLA.

From nanocellulose to wood particles: A review of particle size vs. the properties of plastic composites reinforced with cellulose-based entities

“…Fluorescence microscopy visualization of laminate composites bond lines revealed the ingress of thermopolymer into the pores of the adjacent wood phase, which was found to be temperature-dependent. The degree of ingress of polymer into the pores of the wood was indicative of effective adhesion between the phases (Luedtke et al 2019;Grigsby et al 2019).…”

Chemical Imaging of the Polylactic Acid − Wood Adhesion Interface of Bonded Veneer Products

“…While nascent in industrial uptake, there is significant potential for PLA use in building and construction materials [5]. This is primarily WPCs, but also includes reconstituted and laminated products [6,7].…”

“…Poly (lactic acid) use to bond wood has been demonstrated in a range of product applications, including fiber-, particle-, and veneer-bonded products [6,[8][9][10]. The polymer can be applied as a dry hot-melt or as an emulsion-based adhesive, and be used to achieve interior grade (Type II) property requirements for plywood, for example.…”

“…The adhesion of PLA-based hot melt in wood joints can further be improved using, for example, plasma treatment [12]. As an adhesive, this performance is dependent on composite processing temperatures with distinctions also apparent due to the wood species used in the composite materials [6,7]. At a fundamental level, higher temperatures are associated with lower PLA melt viscosities and the promotion of PLA migration into the wood substrate.…”

“…Differences in adhesive performance may be attributable to the over-penetration of PLA into the wood substrate, akin to that for liquid wood adhesives use [13]. However, the techniques to observe and measure this have so far been restricted to fluorescence microscopy use with a pre-modified PLA polymer [6,7]. If the PLA could be directly analyzed, there is an opportunity to learn more about the adhesion formed at the PLA−wood interface, and the inter-dependencies of the processing variables, wood species, and PLA types on the composite and laminate performance.…”